1. Field of the Invention
This invention relates to an arctic pipeline, having a portion which is under the surface of the permafrost region, for restoring the transmission pressure losses.
2. Description of the Prior Art
The planned Alaskan natural-gas pipeline system may have a length of 9,000 km at a cost of more than 25 billion dollars. A portion of the pipeline will be below the surface of the permafrost. To prevent damage to ecology and to the pipeline itself, the temperature of the gas stream flowing within the pipeline must be continuously maintained at or below 0.degree. C. A higher temperature than 0.degree. C. might warm up the pipeline and melt the surrounding permafrost. The gas stream within the pipeline must also be maintained above a predetermined threshold pressure level to allow continuous gas flow through the pipeline. The gas sustains a transmission pressure loss as it flows south from the northern arctic regions. It is being proposed to restore the pressure loss by recompressing the gas in compressor stations suitably spaced along the length of the line.
Each station is intended to have a compressor powered by a suitable prime mover. The compressor will raise the pressure of the gas to the maximum pressure allowed within the pipeline. Unfortunately, the act of gas compression is accompanied also by an inevitable and undesirable temperature rise. Therefore, the compressed gas discharged by the compressor cannot be re-injected into the pipeline until its temperature is reduced to the temperature of the gas specified for the pipeline, which is typically 0.degree. C. or less.
A considerable amount of research went into developing a process for cooling the warm compressed gas which is discharged by the compressor in the compressor station. It has been proposed by others that this warm compressed gas be cooled by a mechanical refrigeration system utilizing a refrigeration compressor for pumping and compressing a refrigerant fluid. This fluid circulates within a cooling coil. The compressed warm gas is made to pass through the cooling coil, and is cooled to 0.degree. C. or less, which is the temperature of the gas within the pipeline. In the process of cooling the gas stream, the refrigerant fluid warms up. To remove the heat from the refrigerant fluid, it is pumped by the refrigerator compressor to several conventional air coolers.
The use of a mechanical refrigeration system for reducing the temperature of the warm compressed gas discharged by the compressor within the compressor station is technically sound. But, a mechanical refrigeration system is relatively costly to install, maintain, and operate. It consumes a relatively large amount of energy. The refrigeration compressor has to operate not only in the summer time but also in the winter months, at which time the ambient air may be -25.degree. C. or less. Relatively expensive refrigeration spare parts must be maintained for repairing the refrigeration equipment, and such repairs require skilled mechanics. To avoid shutting down the compressor station, it must have a complete standby refrigeration unit, thereby considerably increasing the cost of operating the pipeline.
It is the primary object of the present invention to provide a process and equipment for reducing the temperature of the compressed warm gas discharged by the gas compressor within the compressor station without utilizing a refrigeration system as above described.
The employed temperature reducing process of this invention is relatively economical because it requires less costly machinery, consumes less energy, reduces the frequency of machinery failure, utilizes machinery with less moving parts, and takes full advantage of the ambient air temperature for cooling the warm gas stream discharged from the gas compressor. Thus, in the colder months of the year, the energy consumed for cooling the warm compressed gas stream is at a minimum.